V.O.C.s, NOx and CO have long been a major source of air pollution as an inevitable contaminant exhausted from many industrial processes including, for example, large industrial paint shops used in the automotive industry. Legislative efforts have established emission standards to control emission of such pollutants into the environment. Current and future compliance with such standards places a continuing demand on industry and creates an on-going need to reduce, degrade, and eventually destroy the pollutants in these emissions in a manner that is not cost prohibitive. This is particularly critical for manufacturers in industrialized countries who compete against sources operating in countries that do not have strict air pollution control laws.
In automotive paint shops, large volumes of solvent (V.O.C.) laden air must be removed from the paint spray booths and, to a lesser extent, from other paint shop operations, such as holding and quiet zones and paint bake ovens. For automotive paint shops, large quantities of solvent-laden air must be processed. Various techniques and combinations thereof have heretofore been used for V.O.C. abatement in paint shops.
Typically, scrubbers are used to capture inorganic chemicals and particulate paint from process exhaust air using liquids pumped through the scrubber. Any remaining paint particles are then removed from the exhaust air by filter banks with progressively increasing efficiencies. Less expensive filters are used in the initial stages to trap most of the larger paint particles. After filtering, the exhaust air stream is heated to reduce the humidity for a subsequent adsorption process. In the adsorption processes, solvent-laden air is concentrated into smaller quantities, typically 10% of the main exhaust airstream, and then processed. Typically, the concentration is accomplished by adsorbing the V.O.C.s into a carbon bed and then desorbing the carbon bed with hot air, hot inert gas or steam. The concentrated desorption product can then be finally processed through chemical treatment, solvent recovery or incineration.
Various incineration apparatus can be used to oxidize the solvents in a concentrated solvent-air mixture taken from the carbon beds. However, typically the mixture is heated to temperatures in excess of 1,400.degree. F. When held at these high temperatures, the solvents react with oxygen, with the final reaction production products theoretically being harmless water vapor and carbon dioxide. Various types of thermal regeneration heat exchangers and the like are used to recoup heat from the incinerator exhaust to improve thermal efficiency. Direct incineration could be used but it generally has a low thermal efficiency, particularly for processing large volumes of V.O.C.-laden air. However, such prior art systems require large capacity carbon beds and have high energy costs for incineration.
Except in the most advanced systems, some off-site treatment and/or disposal is frequently required. For smaller installations, as contrasted to large automotive assembly plants, off-site carbon bed desorption may be most cost effective. In general, carbon beds, when used alone, are not effective or cost efficient for processing large volumes of V.O.C.-laden air. Special systems are required to desorb the carbon beds and, for many applications, this is accomplished off-site. Additionally, the desorption concentrate must still be treated for solvent removal and/or incineration. Incineration generally generates NOx or carbon monoxide and, without thermal recovery systems, has a direct thermal impact on the environment as well as requiring off-site disposal. Most importantly, prior art systems relying on high temperatures to complete oxidation are expensive to operate and may still require off-site disposal. These disadvantages, particularly when coupled with current and anticipated air and environmental pollution control, create an on-going need for improved V.O.C. abatement.
Another technique that has been utilized for abatement of V.O.C.s in industrial process air involves the use of ultraviolet (uv) light to break down the V.O.C.s directly and to form activated air containing oxygen in the form of ozone and other oxidants that also work to break down the V.O.C.s. As used herein, "activated air" should be understood to refer to air that has been treated, whether by exposure to ultraviolet light or some other method, to increase the concentration of oxidants in the air. Commercial systems are available that utilize this technique for abatement of solvents contained in process air exhausted from industrial paint booths, ovens, conformal coating areas, etc. A typical system includes a two-stage pre-filter, a photolytic reactor, an aqueous reactor, a coalescer, and a pair of granular carbon beds. Particulates of one micron and greater in size are collected and removed from the process air by the pre-filters. The air flow then passes through the photolytic reactor, where it is exposed to tuned ultraviolet light. Exposure of the process air to the ultraviolet light results in photochemical reactions that form ozone from the oxygen contained in the air, as well as peroxides from the moisture content within the air.
Oxidative degradation begins in the photolytic reactor due to both the newly formed oxidants and the direct exposure of the V.O.C.s to the ultraviolet light. The air stream is then scrubbed with ozonated water in the aqueous reactor. The ozonated water is generated by subjecting air to the ultraviolet lights and then injecting and mixing the activated air into the water. At this stage, water soluble hydrocarbons will collect in the water and will thereby be removed from the air stream. After passing through the aqueous reactor, the water vapor contained in the air stream is removed by the coalsecer. The final stage in this process is to pass the air stream through a carbon bed for adsorption of any remaining V.O.C.s. A second carbon bed is utilized so that while one carbon bed in on-line to adsorb the V.0.C.s, the other is in the process of being regenerated using activated air containing ozone, hydrogen peroxide, and other oxidants produced photochemically by exposure of clean air to ultraviolet light.
The use of ultraviolet light to generate activated air containing ozone has also been implemented in various systems for treating water. For example, the following U.S. Pat. Nos. are each directed to the use of ozone and other oxidants in the wash water of a laundry washing system: 3,065,620, issued Nov. 27, 1962 to P. H. Houser; 3,130,570, issued Apr. 28, 1964 to P. M. Rentzepis; 3,194,628, issued Jul. 13, 1965 to P. Cannon; 5,097,556, issued Mar. 24, 1992 to R. B. Engel et al.; and 5,241,720, issued Sep. 7, 1993 to R. B. Engel et al. In these systems, ozone is produced by exposing air to ultraviolet radiation that is produced by either a corona discharge or ultraviolet lamps. The activated air containing ozone and, in some cases, hydrogen peroxide is mixed with the wash water to improve the cleaning of laundry and reduce or even eliminate the need for detergents.
The literature also suggests that substantial laboratory efforts have been directed to using ultraviolet radiation for other types of water treatment. See Legrini, Oliveros and Braun, "Photochemical Processes For Water Treatment," Chem. Rev. 1993 at pages 671 through 698, American Chemical Society Document No. 0009-2665/93/0793-0871. Ultraviolet radiation for water treatment is potentially useful not only for treating drinking water, but also for treating contaminated surface water, ground water and waste water. However, based upon the 221 biographical references cited and reviewed, the authors suggest that most such laboratory experimentation, with a few noted exceptions, have not been evaluated on a prototype basis, much less commercially.
Although the Chemical Review article is directed to water treatment as contrasted to V.O.C. abatement in industrial process air, some of the mechanics of oxidative degradation considered therein may be useful as background for the present invention. For example, Table I at page 674 (reproduced as "TABLE 1" below) confirms the oxidation potential of various oxidants believed to be available from the activated air and undoubtedly generated elsewhere in the system and process of the present invention as will be described.
TABLE 1 ______________________________________ Oxidation potentials of Some Oxidants Oxidation Potential Species (V) ______________________________________ fluorine 3.03 hydroxyl radical 2.80 atomic oxygen 2.42 ozone 2.07 hydrogen peroxide 1.78 perhydroxyl radical 1.70 permanganate 1.68 hypobromous acid 1.59 chlorine dioxide 1.57 hypochlorous acid 1.49 hypoidous acid 1.45 chlorine 1.36 bromine 1.09 iodine 0.54 ______________________________________
Each of the foregoing references, as well as the literature describing the commercially available air treatment systems described above, all espouse the virtues of ozone and the use of ultraviolet radiation to generate that ozone. However, as shown in Table 1 above, ozone has a lower oxidation potential than hydroxyl radicals. Thus, ozone has less tendency to cause oxidation of the V.O.C.s than the hydroxyl radical; that is, it is less active than the hydroxyl radical. U.S. Pat. No. 4,214,962, issued Jul. 29, 1980 to A. J. Pincon, sets forth other disadvantages of creating ozone in addition to other oxidants formed by ultraviolet radiation; namely, the increase in surface tension of water with which it is mixed and the possible formation of carcinogenic substances. In that patent, an apparatus is disclosed for using ultraviolet light under 200 nanometers to generate an undisclosed activated oxygen product without the production of ozone. When used for treating water for human consumption or swimming pools, the apparatus can include a polyvinyl chloride enclosure to permit liberation of free chloride to provide chlorination of the water.
Not unexpectedly, however, since the Pincon patent is directed to the use of ultraviolet radiation for water treatment, it does not address the problems associated with processing large quantities of industrial process air laden with V.O.C.s, much less offer any direct solution to the problems and disadvantages of the various commercial processes that rely at least in part on the presence of ozone for V.O.C. abatement in the exhaust from paint spray booths.
There are many other industrial processes that produce exhaust air containing V.O.C.s, NOx, and CO. Such processes are utilized in the automotive, wood products, laundry dryers, furniture manufacture, foundries, plastic, stationary diesel and turbine generators, power plants, tanning, inks and printing, paper products, paper mills, and refineries.
Many of these industries utilize processes that generate large amounts of NOx. For example, in the wood products industry, dryers used in the manufacture of pressboard often burn available materials such as saw dust and wood chips to provide the heat needed for drying the pressboard. As a result, large amounts of NOx are produced. To help reduce the amount of NOx contained in exhaust air from these dryers, incineration is sometimes used. However, this process still results in thermal pollution and has only limited ability to reduce and destroy NOx in the exhausted air.